Drill a 1/4 (6mm) hole 1 inch (50mm) from the end of each header pipe and bolt it in with a couple of lock nuts
Now the washers can be rotated to alter the back pressure and allow you to fine tune.
If you are going to do this on a system with a free-flowing silencer then drill the holes just in front of the connection of the silencer to the header(s).
This is supposed to be an old Honda factory works trick that should work on any system.
